Compressor discharge bleed air circuit in gas turbine plants and related method

ABSTRACT

A gas turbine system that includes a compressor, a turbine component and a load, wherein fuel and compressor discharge bleed air are supplied to a combustor and gaseous products of combustion are introduced into the turbine component and subsequently exhausted to atmosphere. A compressor discharge bleed air circuit removes bleed air from the compressor and supplies one portion of the bleed air to the combustor and another portion of the compressor discharge bleed air to an exhaust stack of the turbine component in a single cycle system, or to a heat recovery steam generator in a combined cycle system. In both systems, the bleed air diverted from the combustor may be expanded in an air expander to reduce pressure upstream of the exhaust stack or heat recovery steam generator.

BACKGROUND OF THE INVENTION

[0001] In some gas turbine applications, there are instances of gasturbine plant operation where the gas turbine pressure ratio reaches theoperating pressure ratio limit of the compressor, resulting incompressor surge. These instances may arise in applications wherelow-Btu fuels or any other fuels with large amounts of diluent injectionare used, and/or at cold ambient temperature conditions. The compressorpressure ratio is typically larger than the turbine pressure ratio inthat the latter is subject to pressure loss in the turbine combustor.

[0002] One common solution that has been used to provide compressorpressure ratio protection is the bleeding off of gas turbine compressordischarge air and recirculating the bleed air back to the compressorinlet. This method of gas turbine operation, known as Inlet Bleed Heat(IBH) Control, raises the inlet temperature of the compressor inlet airby mixing the colder ambient air with the bleed portion of the hotcompressor discharge air, thereby reducing the air density and the massflow to the gas turbine. While this approach eliminates compressorsurge, it also reduces turbine output both for the simple cycleoperation as well as for combined cycle operation. In the latter case,the reduced gas turbine exhaust flow produces less steam in the HeatRecovery Steam Generator (HRSG) and consequently less steam turbineoutput. IBH also reduces the thermal efficiency of the gas turbine dueto the loss of energy in throttling the compressed air.

BRIEF SUMMARY OF THE INVENTION

[0003] This invention provides an improved compressor bleed air methodfor providing compressor pressure ratio protection, which results inimproved output and efficiency of a simple or combined cycle gas turbinepower plant (as compared to the IBH approach). This invention is mostly,but not specifically, applicable to gas turbines utilizing standarddiffusion flame combustors.

[0004] Two embodiments are disclosed herein. Each has applicability toboth simple and combined cycle systems.

[0005] In a first embodiment, the invention includes bleeding off enoughgas turbine compressor discharge air to maintain the compressor pressureratio limit, and mixing it with the gas turbine (GT) exhaust in a simplecycle system, or at an appropriate location in a combined cycle system(e.g., in the HRSG stack where the two streams have minimum temperaturedifference). This technique does not increase compressor inlettemperature, and thus does not reduce output as in the case of the IBHapproach.

[0006] In a second embodiment, where the compressor bleed function isactivated for a large percentage of the gas turbine operating period,the method is similar to that described above, except that it uses anair expander device to recover the excess energy associated with thedifference between the compressor air discharge pressure and GT exhauststack (or HRSG) pressure. In addition to the power output increases,this method also results in higher power plant efficiency. In thissecond embodiment, a portion of the compressor discharge bleed air maybypass the expander via a throttling device to combine with thedischarge stream from the expander, to thereby enable plant operationduring start up, shut down and during the events when the expander isnot operating.

[0007] The following are additional optional modifications which may beselected (individually or in an appropriate combination, in both simpleand combined cycle operations) based on the economic benefits for agiven application. The high pressure bleed air from the compressor isfurther heated, if required, by means of a pre-heater prior tointroduction in the air expander to improve the expander output. Thesource of this heat can be thermal energy recovered either upstream,such as in the example case of a gasifier with high temperature cooler,or downstream such as the exhaust gas heat recovered from the gasturbine exhaust in a waste heat boiler. Alternatively, the source ofheat may include combustion of air and fuel separately supplied to thepre-heater.

[0008] In its broader aspects, therefore, the invention relates to asimple cycle gas turbine system comprising: a compressor, a turbinecomponent and a load, wherein fuel and compressor discharge bleed airare supplied to a combustor and gaseous products of combustion areintroduced into the turbine component and subsequently exhausted toatmosphere; and a compressor discharge bleed air circuit that removesbleed air from the compressor and supplies one portion of the bleed airto the combustor and another portion of the bleed air to an exhauststack of the turbine component.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic diagram of a simple cycle gas turbine withcompressor bleed air to an exhaust stack in accordance with theinvention;

[0010]FIG. 2 is a schematic diagram of a simple cycle gas turbine withcompressor bleed air pressure energy recovery in accordance with theinvention;

[0011]FIG. 3 is a schematic diagram of a combined cycle gas turbine withcompressor bleed air to a heat recovery steam generator; and

[0012]FIG. 4 is a schematic diagram of a combined cycle gas turbine withcompressor bleed air pressure energy recovery.

DETAILED DESCRIPTION OF THE INVENTION

[0013] With reference to FIG. 1, the simple cycle gas turbine system 10includes a compressor 12, a turbine component 14 and a load (e.g., agenerator) 16 arranged on a single rotor or shaft 18. A combustor 20 ofthe gas turbine receives fuel via stream 22 and control valve 24, aswell as hot discharge air bled off from the compressor 12 via stream 26.Combustion gases are introduced into the turbine component 14 via stream28.

[0014] During potential compressor surge conditions, a compressordischarge bleed air circuit is utilized. This circuit causes some of thecompressor discharge bleed air to bypass the combustor and directs thebleed air directly to the gas turbine exhaust stack 30 via stream 32 andthrottle valve 34, the valve 34 also controlling the amount of bleed airintroduced into the combustor 20.

[0015] By extracting sufficient compressor discharge bleed air andfeeding it directly to the gas turbine exhaust stack 30, the compressorpressure ratio limit is protected while, at the same time, there is noincrease in the compressor inlet air temperature, and thus no loss ofturbine output.

[0016] In FIG. 2, an arrangement is illustrated that is particularlybeneficial when the compressor bleed air function is employed for alarge percentage of the gas turbine operating period. In thisembodiment, the gas turbine system 36 includes a compressor 38, aturbine component 40 and a load (e.g., a generator) 42, arranged on asingle rotor or shaft 44. The combustor 46 receives fuel via stream 48and fuel control valve 50; and compressor discharge bleed air from thecompressor 38 via stream 52. Combustion gases are introduced into theturbine component 40 via stream 54. A predetermined percentage of thecompressor discharge bleed air is directed to a flow control/bypassvalve 56. During potential compressor surge conditions, valve 56supplies compressor discharge bleed air to an air expander 68 via stream70, and the air representing the difference between the compressor airdischarge pressure and the gas turbine exhaust pressure, is then used todrive a secondary load 72 (e.g., a generator) via shaft 74. Optionally,the valve 56 can adjustably divert the compressor discharge bleed air tothe gas turbine exhaust stack 58 via stream 60 and flow control throttlevalve 62. This is useful as a bypass scheme (bypassing expander 68) tocontinue plant operation during start-up, shut-down and other eventswhen the expander is not operating.

[0017] In an alternative arrangement, valve 56 may supply the compressordischarge bleed air to a pre-heater 64 via stream 66. Pre-heater 64heats the bleed air by heat exchange with turbine exhaust air fed to thepre-heater 64 via stream 76. The heated bleed air is then expanded asdescribed above. The pre-heater 64, optionally, may be fired using fuelseparately introduced via stream 78. Excess air from the expander 68 isintroduced into stream 60 via stream 80 and then to the gas turbineexhaust stack 58. Some percentage of this excess air may be allowed tobypass the stack 58 and escape to atmosphere upstream of the stack 58via stream 81 and valve 82.

[0018] Turning now to FIG. 3, a combined cycle system 84 includes a gasturbine including a compressor 86, a turbine component 88 and a load(e.g., a generator) 90 arranged on a single shaft 92. Combustor 94receives fuel via stream 96 and fuel control valve 98 along withcompressor discharge air bled off from the compressor 86 via stream 100.The gas turbine exhaust is supplied via stream 102 to a heat recoverysteam generator (HRSG) 104 for reheating steam from a steam turbine 106.Condensed steam from steam turbine 106 is supplied to the HRSG 104 viastream 108 and the reheated steam is returned to the steam turbine viastream 110. Steam turbine 106 drives a second generator 107 via shaft112.

[0019] During potential compressor surge conditions, a portion of thecompressor discharge bleed air is supplied to the HRSG 104 via stream114 and flow control/throttle valve 116, where it mixes with the gasturbine exhaust before being released to atmosphere via the HRSG exhauststack 118. As in the embodiment shown in FIG. 1, this arrangement doesnot result in an increase in compressor inlet air temperature, thusallowing the compressor to enjoy the full benefit of low ambienttemperatures (or other factors that also produce compressor surge).

[0020] Turning now to FIG. 4, an arrangement is shown that is applicableto combined cycle systems and uses an air expander to recover energyassociated with the difference between the compressor discharge airpressure and the HRSG pressure. This combined cycle system 120 includesa gas turbine having a compressor 122, a turbine component 124 and aload (e.g., a generator) 126 arranged on a single shaft 128. Combustor130 receives fuel via stream 132 and fuel control valve 134, along withcompressor discharge air bled off from the compressor 122 via stream136. Combustion gases from the combustor 130 are introduced into theturbine 124 via stream 138. The gas turbine exhaust is supplied viastream 140 to an HRSG 142 for reheating steam from the steam turbine144. Condensed steam from the steam turbine 144 is supplied to the HRSG142 via stream 146, and the reheated steam is returned to the steamturbine 144 via stream 148. Steam turbine 144 drives a generator 145.

[0021] During potential compressor surge conditions, a predeterminedpercentage of the compressor discharge bleed air is directed to a flowcontrol/bypass valve 150 via stream 152. Valve 150 supplies the bleedair to the expander 154 via stream 156. Optionally, the bleed air canfirst be supplied to a pre-heater 158 via stream 156. The pre-heater 158heats the compressor discharge bleed air via heat exchange with gasturbine exhaust in the HRSG via stream 160. The pre-heater 158,optionally, may be fired using fuel introduced via stream 162. Theheated compressor discharge bleed air is then expanded in the airexpander 154 and the excess air is used to drive a third load (e.g., agenerator) 164 via shaft 166. During start-up, shut-down or other eventswhen the expander is not operating, the valve 150 may divert thecompressor discharge bleed air to the HRSG 142 via stream 166 and flowcontrol/throttle valve 168, thus bypassing the pre-heater 158 andexpander 154. When in service, air from the expander 154 is dumped intothe stream 166 upstream of the HRSG 142, via stream 170. It iseventually exhausted to atmosphere through the HRSG stack 172. Somepercentage of this air may bypass the HRSG 142 and escape to atmospherevia stream 174 and valve 176.

[0022] It is significant that the compressor discharge bleed aircircuits described above are useful under conditions that lead tocompressor surge, i.e., low ambient air temperatures; excess flow to theturbine; use of fuels with low heat content, etc. By channeling thecompressor bleed air downstream of the compressor, there is no increasein compressor inlet temperature and attendant loss of input as with theIBH approach. With higher ambient temperatures, flow is reduced andthere is typically no danger of compressor surge, so that the bleed airtechniques of this invention are not required.

[0023] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A simple cycle gas turbine system comprising: acompressor, a turbine component and a load, wherein fuel and compressordischarge bleed air are supplied to a combustor and gaseous products ofcombustion are introduced into the turbine component and subsequentlyexhausted to atmosphere; and a compressor discharge bleed air circuitthat removes bleed air from the compressor and supplies one portion ofthe bleed air to the combustor and another portion of the bleed air toan exhaust stack of the turbine component.
 2. The system of claim 1including a throttle valve for controlling flow of the bleed air to theexhaust stack.
 3. The system of claim 1 wherein said compressor, turbinecomponent and load are on a single shaft.
 4. The system of claim 1wherein said compressor discharge bleed air circuit includes a flowcontrol/bypass valve enabled to divert said another portion of the bleedair to an expander.
 5. The system of claim 4 wherein said anotherportion of the bleed air is introduced into a pre-heater upstream ofsaid expander.
 6. The system of claim 4 wherein said another portion ofthe bleed air discharged from said expander is introduced into saidexhaust stack.
 7. The system of claim 4 wherein said another portion ofthe bleed air is bypassed around the expander to enable turbineoperation during start-up and shut-down.
 8. A combined cycle systemcomprising a compressor, a gas turbine component, a first load, a heatrecovery steam generator and a steam turbine; wherein fuel andcompressor discharge bleed air are supplied to a combustor and gaseousproducts of combustion are introduced into the gas turbine component andsubsequently exhausted to said heat recovery steam generator forre-heating condensed steam from said steam turbine and returning thereheated steam to said steam turbine for driving a second load; and acompressor discharge bleed air circuit that supplies one portion of thebleed air to the combustor and another portion of the bleed air to theheat recovery steam generator.
 9. The system of claim 8 including athrottle valve for controlling flow of bleed air to the heat recoverysteam generator.
 10. The system of claim 8 wherein said compressor,turbine component and load are arranged on a single shaft.
 11. Thesystem of claim 8 wherein said compressor discharge bleed air circuitincludes a flow control/bypass valve enabled to divert bleed air to anexpander upstream of said exhaust stack.
 12. The system of claim 11wherein the bleed air is introduced into a pre-heater upstream of saidexpander.
 13. The system of claim 12 wherein bleed air discharged fromsaid expander is introduced into an exhaust stack of said heat recoverysteam generator.
 14. A gas turbine operating system comprising acompressor, a turbine component and a load wherein fuel and compressordischarge bleed air are supplied to a combustor and gaseous products ofcombustion are introduced into the turbine component; and a compressordischarge bleed air circuit comprising means for avoiding compressorsurge under low ambient temperature conditions without increasingcompressor inlet air temperature.
 15. A method of avoiding compressorsurge under low ambient temperature conditions in a gas turbineoperating system that includes a compressor, a turbine component and aload, the method comprising: a. bleeding discharge air from thecompressor and supplying a portion of the bleed air to a combustor ofthe gas turbine; and b. supplying another portion of the bleed air to anexhaust stack of the turbine component to thereby avoid compressor surgewithout increasing compressor inlet air temperature.
 16. The method ofclaim 15 including, prior to step b., supplying said another portion ofthe bleed air to an expander upstream of said exhaust stack.
 17. Themethod of claim 16 including bypassing the bleed air around the expanderduring startup and shut down.
 18. The method of claim 16 including usinga portion of the air exiting the expander to drive another load.
 19. Amethod of avoiding compressor surge under low ambient temperatureconditions in a combined cycle gas turbine operating system thatincludes a compressor, a turbine component, a generator, a heat recoverysteam generator and a steam turbine wherein exhaust from the gas turbinecomponent is used to reheat condensed steam from the steam turbine inthe heat recovery steam generator, the method comprising: a. bleedingdischarge air from the compressor and supplying a portion of the bleedair to a combustor of the gas turbine; and b. supplying another portionof the bleed air to the heat recovery steam generator to thereby avoidcompressor surge without increasing compressor inlet air temperature.20. The method of claim 19 including, prior to step b., supplying saidanother portion of the bleed air to an expander upstream of said heatrecovery steam generator.
 21. The method of claim 20 including bypassingthe bleed air around the expander during startup and shut down.
 22. Themethod of claim 19 including using a portion of the air exiting theexpander to drive another load.